chore: set native issue types in the issue forms (#85)

The forms already stamp a title prefix and a label, but not the native
GitHub issue type, so every newly-filed ticket landed untyped and the
Epic/Story hierarchy decayed one issue at a time.

Adds `type: Epic|Story|Task` so the taxonomy is enforced at creation
rather than reapplied by hand afterwards.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>